Shoppers walk toward the San Ysidro, Calif., Port of Entry after making purchases at outlet malls along the US-Mexico border on Dec. 29.

Updated Jan. 25 at 9:05 a.m. ET

The Trump administration on Friday is implementing its plan requiring asylum-seekers, mainly from Central America, to remain in Mexico while their legal proceedings are conducted in the U.S. court system.

The policy will be rolled out at San Diego’s San Ysidro border crossing, the nation’s busiest, a senior official with U.S. Customs and Border Protection tells NPR. The plan, coming out of talks with Mexican officials, is to bus asylum-seekers back and forth from Tijuana, Mexico, to a courthouse in downtown San Diego. The administration plans to eventually implement the policy at other border crossings.

Homeland Security Secretary Kirstjen Nielsen first announced the plans for this “historic” policy, called the “Migration Protection Protocols,” in a memo issued in late December.

“Aliens trying to game the system to get into our country illegally will no longer be able to disappear into the United States, where many skip their court dates. Instead, they will wait for an immigration court decision while they are in Mexico. ‘Catch and release’ will be replaced with ‘catch and return,'” said Nielsen.

“Catch and release” is a term the Trump administration uses for the traditional policy of allowing asylum-seekers to remain in the U.S. pending a court hearing.

A “Fact Sheet” issued by the Department of Homeland Security to explain the new policy says asylum seekers “will be given a ‘Notice to Appear’ for their immigration court hearing and will be returned to Mexico until their hearing date.”

“While aliens await their hearings in Mexico, the Mexican government has made its own determination to provide such individuals the ability to stay in Mexico, under applicable protection based on the type of status given to them.

“Aliens who need to return to the U.S. to attend their immigration court hearings will be allowed to enter the U.S. and attend that hearing. Aliens whose claims are found meritorious by an immigration judge will be allowed to remain in the U.S. Those determined to be without valid claims will be removed from the U.S. to their country of nationality or citizenship.

“DHS is working closely with the U.S. Department of Justice’s Executive Office for Immigration Review to streamline the process and conclude removal proceedings as expeditiously as possible.”

The new policy, which has been dubbed “Remain in Mexico” by administration officials and critics alike, will not apply to children traveling alone, known as unaccompanied minors, or to asylum-seekers from Mexico.

A senior official with Customs and Border Protection told NPR he expects the new policy will be met with a swift legal challenge.

Immigrant and asylum advocates were quick to denounce the policy change.

“Forcing families seeking asylum to remain in dangerous conditions in Mexico spits in the face of the values at the core of our country’s identity,” said Jess Morales Rocketto, chair of Families Belong Together, a coalition of 250 groups opposing the administration’s immigration policies. “The reality of this plan is suffering. These children and families will be left unsafe, without food or shelter, in violation of their human rights.”